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M. Allen (1) (1) University of Oxford, School of geography and the environment, Oxford, United Kingdom

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The relative priority given to reducing carbon dioxide emissions versus other forms of anthropogenic climate pollution, such as the Short Lived Climate Pollutants (SLCPs) methane and soot, could be a significant issue for many countries in the preparations for COP21. The decision to allow countries to adopt their own metrics to add up the role of different gases in their contributions to the overall emission reduction goal highlights the need for clarity on these issues. This talk will review the findings on cumulative carbon and emission metrics in the IPCC 5th Assessment, and present a new Policy Brief from the Oxford Martin School, with the same title, that discusses and elucidates these issues.

Any strategy to prevent dangerous anthropogenic interference in the climate system must limit cumulative emissions of the main long-lived climate pollutant, carbon dioxide (CO2). To limit the warming they cause to 2°C, for example, CO2 emissions must be limited to a cumulative budget of about one trillion tonnes of carbon, over half of which has already been released. That said, other climate drivers are likely to contribute to peak warming, and hence reduce the carbon budget consistent with 2°C of total anthropogenic warming. Current emissions of both CO2 and SLCPs also affect the rate and magnitude of climate change over the next few decades, although it is important to note that the climate benefits of reduced emissions on these short timescales could be comparable to, and hence potentially outweighed by, natural climate variability, particularly on regional scales.

Reductions in SLCP emissions could be achieved at relatively low cost and with substantial co-benefits but, I will argue, implementing these reductions immediately would have little impact on peak warming unless CO2 emissions are substantially reduced at the same time. Hence any decisions on policy priorities between CO2 and SLCPs, and any country's choice of emission metric to relate them, represents in essence a matter of intergenerational prioritisation: Advancing measures to reduce SLCP emissions could provide some climate benefit to the current generation through reduced warming over the next few decades, while immediate reductions in CO2 emissions also deliver a more substantial climate benefit to future generations.

Any emission trading system or climate policy that addresses emissions of several different greenhouse gases together in a single ‘multi-gas basket’ requires some form of metric to specify what a given amount of one greenhouse gas is ‘worth’ in terms of another. The choice of metric to compare the impact of emissions of methane and other SLCPs with the impact of CO2 depends on the timescale of interest. If the policy goal is to limit peak warming, it also depends on the ambition and success of future mitigation measures.

The standard ‘100-year Global Warming Potential’ metric (GWP100) provides (despite its name) an approximate indication of the relative importance of emissions of different gases to the increase in global temperatures over the next 20 to 40 years. GWP100 is therefore a measure of impact on peak warming if and only if temperatures are expected to be approaching stabilization within 40 years, for which CO2 emissions need to approach zero on a comparable timescale.

As long as CO2 emissions continue to rise, policies that allow SLCP measures to be exchanged, traded or offset against CO2 emission reductions using GWP100 over-value the impact of SLCP measures on peak warming and hence risk discouraging the CO2 emission reductions that are required to stabilize temperatures. Replacing GWP100 with a different metric would not solve this problem because any metric that is suitable for long-term impacts would be misleading for short-term impacts and vice versa. Using a metric that changes over time would help, but introduces greater complexity and uncertainty.

Rather than discussing a change of metric within a single ‘multi-gas basket’ framework, policymakers should focus instead on introducing additional instruments, safeguards or measures to ensure that cumulative emissions of CO2 are limited to an overall budget consistent with meeting the 2°C goal. A simple precaution would be to avoid trading or offsetting between CO2 and SLCP emission reductions until global CO2 emissions are falling fast enough to allow a robust and realistic assessment of the remaining time to peak warming.

The changing face of global fossil fuel carbon dioxide emissions

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Global fossil fuel carbon dioxide emissions have been persistently increasing for the last 250 years. This increase is a primary driver of the atmospheric disequilibrium impacting and changing atmospheric, terrestrial, and oceanic systems.

The Carbon Dioxide Information Analysis Center (CDIAC) at Oak Ridge National Laboratory (ORNL), U.S.A., has been estimating fossil fuel carbon dioxide emissions for more than 20 years. These emission estimates are based on fuel statistics, carbon contents, and the fraction of fuel oxidized. This has resulted in an annual time series of emission estimates from the year 1751 to the present. Annual updates add another year to the time series as well as revising data in previous years. Over the years, this basic time series has been supplemented by mapping the emissions at one degree latitude by one degree longitude, describing the time series in terms of stable carbon isotopic (ä13C) signature, parsing the time series from annual to monthly time steps, and describing the uncertainty of the global total FFCO2 emissions. Underway now is an uncertainty evaluation of the annual and monthly mapped emissions.

This time series reveals interesting trends when disaggregated by country, fuel type, and source. For example, the proportion of emissions from Kyoto Protocol Annex B and non-annex B countries has changed since Protocol signing to today. The role of coal in fueling global energy systems has changed from being the primary fuel to becoming secondary to liquid fuels to again becoming the primary fuel. The practice of flaring excess gas in oil fields, as a proportion of total energy production, has decreased by more than a factor of two since the year 1950. Release of carbon during cement production has increased by more than a factor of four since 1950 and now equates to about 5% of global carbon dioxide emissions from fossil fuel production.

The global fossil fuel carbon dioxide uncertainty analysis resulted in a 2σ range of 1.0 to 13%, which can be greatly simplified to 8.4% (2σ). This uncertainty in the magnitude of global fossil fuel carbon dioxide emissions has become an important component to our overall understanding of the global carbon cycle. The uncertainty in the magnitude of mapped fossil fuel carbon dioxide emissions will become a limitation to our understanding of local carbon cycles in the absence of detailed local inventories and observations.

The emission time series has been used by the Global Carbon Project in its annual evaluation of the global carbon budget. The budget is a high level check on the understanding of carbon flows throughout the Earth system. Uncertainty propagation through the budget constrains our knowledge about the carbon system.

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Potential emissions of CO2 and methane from proven reserves of fossil fuels in the context of the global remaining carbon budget

R. Heede (1) ; N. Oreskes (2) (1) Climate Accountability Institute, Snowmass, CO, United States of America; (2) Harvard University, and Climate Accountability Institute, Department of the history of science, Cambridge, MA, and Snowmass, CO, United States of America

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Scientists have argued that no more than one-third of proven recoverable fossil fuel reserves can be extracted and consumed by 2050 if we are to avoid exceeding the 2ºC temperature target agreed to in Copenhagen (IEA 2012). The remaining carbon budget is ≤275 gigatonnes of carbon (GtC) (assuming a 66% probability of staying below 2ºC; IPCC 2014), whereas recoverable fossil fuel reserves contain an estimated 733 GtC (based on BP 2014; IPCC 2014 cites reserves of 1.0-1.9 TtC). Global reserves are based on national assessments without reference to reserve ownership or productive capacity. The studies that do identify corporate owners are limited to investor-owned companies listed on major securities exchanges. We identify the largest seventy investor-owned and state-owned companies that possess the financial and technical capacity (and the ownership or production rights) to exploit and produce the majority of the world’s recoverable reserves of oil, natural gas, and coal. We quantify the potential emissions of CO2 and methane for each entity’s reserves, and compare the emissions to the global remaining carbon budget.

This presentation will report on recent estimates of the potential emissions of CO2 and methane from the proven reserves declared by the world's largest producers of oil, natural gas, and coal, focusing on the seventy companies and eight government-run industries that produced 63 percent of the world's fossil fuels from 1750 to 2010 (Heede 2014). Full production of these reserves (accounting for non-energy uses and flared and vented CO2) is estimated to result in emissions of 440 GtC of carbon dioxide — or 160 percent of the remaining 275 GtC carbon budget. Of the 440 GtC attributed, the 42 investor-owned oil, gas, and coal companies hold reserves with potential emissions of 44 GtC, whereas the 28 state-owned entities possess reserves of 210 GtC — equivalent to 16 percent and 76 percent of the remaining carbon budget, respectively. Government-run industries possess reserves of 185 GtC (16% of the remaining budget).

This analysis shows that 1) the profound risk to the future arises not so much from the proven reserves in the hands of publicly-traded corporations, but from their on-going exploration for and development of new fossil fuel resources, and 2) that while the investor-owned companies may be most vulnerable to investor and consumer pressure, effective action to control climate change must also include the state-owned companies and governments that hold the preponderance of reserves. This work will inform climate scientists, energy and emission scenario modelers, climate negotiators, national climate and energy policy leaders, and investors in fossil fuel companies with practical data on the potential consequences of fossil fuel reserves held by specific companies with the capacity, financial resources, and incentives to extract, refine, and market carbon fuels. These supra-national companies may play a critical role in delivering solutions to reduce net emissions from the fossil fuel sector.

Net Carbon gain and loss in the world's tropical vegetation: New estimates for the 2012 – 2014 period

A. Baccini (The Woods Hole Research Center, Woods Hole, United States of America), W. Walker (The Woods Hole Research Center, Woods Hole Research Center, United States of America), L. Carvalho (Boston University, Boston, United States of America), D. Sulla-Menashe (Boston University, Boston, United States of America), R. A. Houghton (The Woods Hole Research Center, Woods Hole Research Center, United States of America)

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Net Carbon gain and loss in the world's tropical vegetation: New estimates for the 2012 – 2014 period

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Tropical forests store 471 Pg C, 55 % of global forest stocks, and emissions from land use and land cover change (LULCC) are the second largest anthropogenic source of carbon dioxide to the atmosphere. Despite the near convergence of emissions estimates for the tropics as a whole, estimates for individual regions are much more variable, particularly at national and subnational scales. Forest disturbance and successional processes determine the magnitude and distribution of sources and sinks of atmospheric carbon dioxide, which influence climate. Tracking terrestrial carbon fluxes and predicting how tropical forests will respond to continuous global change relies on accurate estimates of annual changes in the density and distribution of carbon stocks at local to global scales. Thus, determining the magnitude and distribution of sources and sinks at annual time steps with measurable uncertainty is of scientific and political importance. While significant progress has been made in the quantification of single-epoch carbon storage across large areas of the globe, robust assessments of aboveground carbon dynamics remain lacking. Existing evidence for tropical forests as a carbon sink is based on a limited number of repeated in situ measurements that have been scaled to characterize sequestration dynamics across large regions of the tropics. Advancing the work of Baccini et al. (2012), we combine wall-to-wall satellite image data, Light Detection and Ranging (LiDAR) measurements and field data to empirically examine aboveground biomass dynamics (i.e., gains and losses) and quantify net changes in carbon density (i.e., rates of carbon sequestration and emissions) at annual intervals for the period 2002 to 2014 using a hierarchical statistical model that segments annual time series measurements of carbon density according to piecewise linear trends. Thirteen years (2002-2014) of pantropical satellite data serve to provide direct, measurement-based evidence that the world's tropical forests are a net carbon source on the order of 420 Tg C yr -1. This net release of carbon consists of net losses of 548 Tg C yr-1 and net gains of 128 Tg C yr-1. The gains result from forest growth, afforestation and reforestation; losses result from natural disturbance processes as well as both anthropogenic reductions in forest area attributed to deforestation and in biomass density within forests resulting from degradation. While the changes are widely distributed throughout the tropics, the forests of tropical America account for 75 % of the net loss. Advantages of this new approach to emissions estimation over traditional methods that rely on emission factors (carbon density) and activity data (forest area change) include (1) providing spatially explicit, consistent and accurate estimates of net emissions from forest biomass change that eliminate the need for error prone land cover classifications or area change products, (2) accounting for gains and losses without the need to explicitly define or delineate forest degradation and (3) tracking of changes in net forest carbon emissions on an annual basis with quantified uncertainty that is suitable for use in national and international policy making on REDD+ and associated mitigation actions.

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Methane (CH4) is a greenhouse gas (GHG) with the second largest global radiative forcing contribution after CO2. Its atmospheric abundance has increased by about 150% since the industrial revolution. Given the critical role of CH4 in global climate change, it is important to better understand its sources and sinks. Improving the estimates of individual source magnitudes – such as fossil fuels, agriculture, and other anthropogenic and natural sources – helps prioritizing CH4 emissions mitigation efforts and modeling future climate change. Policy-makers rely strongly on accounting-based national and international emissions inventories to inform mitigation regulatory action. However, recent field studies indicate that emissions inventories may significantly underestimate fossil fuel CH4 emissions (those associated with extraction and use of natural gas, oil, and coal). In this work, atmospheric measurements from the National Oceanographic and Atmospheric Administration (NOAA) Global GHG Reference Network spanning the past three decades are used to derive long-term estimates of global CH4 emissions from fossil fuels and other sources in comparison with inventories and other estimates. Atmospheric measurements include global CH4 and the stable isotope 13CH4, which are used in a global box-model to constrain source magnitudes. First, probability distribution functions of the key model parameters are derived including literature isotopic source signatures, isotopic fractionation factors, atmospheric CH4 lifetime, and fossil fuel hydrocarbon gas compositions. Our isotopic source signature distributions are based on the largest literature survey to date, which suggests significant corrections compared to previous studies. Second, a Monte Carlo simulation of the box-model is performed to quantify confidence intervals of individual emissions sources. We find that attributing the majority of increased CH4 levels over the past three decades to microbial sources is consistent with 13CH4. The sum of CH4 emissions from the fossil fuel industry and geological seepage is significantly larger than previous estimates, which is compatible with pre-industrial isotopic ice core records. Third, recently published estimates of global CH4 emissions from oil and coal production are subtracted from our global fossil fuel CH4 results to quantify global CH4 leakage from the natural gas industry during extraction, processing, transport, and distribution of the fuel. Natural gas CH4 leakage as a fraction of total production has decreased steadily over the same period indicating industry efficiency improvements. The results highlight a major gap in our understanding of global CH4 emissions. This motivates increased collaboration efforts between the physical sciences and regulatory agencies to explain and reconcile the differences between atmospheric measurements and emissions inventories.

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Black carbon (BC) in smoke plumes in Northern Eurasia can be transported and deposited on Arctic ice and accelerate ice melting during certain times of the year. Thus, we examined daily BC emissions from fires in this region at a 500 m x 500 m resolution from 2002 to 2012 and modeled the BC transport and deposition in the Arctic. Black carbon emissions were estimated based on MODIS land cover maps and detected burned areas, the Forest Inventory Survey of the Russian Federation, and biomass specific BC emission factors. Annual burned areas in Northern Eurasia varied considerably with an average of 260,000 km2 for the study period. Grassland dominates the total burned area (61%), followed by forest (27%). For grassland fires, about three-quarters of the area burned occurred in Central and Western Asia and about 17% in Russia. More than 90% of the forest burned area was in Russia. Annual BC emissions from Northern Eurasian fires varied enormously with an average of 0.85 Tg. In contrast to burned area, BC emissions from forest fires accounted for about two-thirds of the emissions, followed by grassland fires (15%). More than 90% of the BC emissions from forest fires occurred in Russia. Central and Western Asia is the major region for BC emissions from grassland fires (53%). Overall, Russia contributed 83% of the total BC emissions from fires in Northern Eurasia.

The transport and deposition of BC on Arctic ice from all the global sources was estimated using the LMDz-OR-INCA global chemistry-aerosol-climate model at the LSCE. Overall, about 55% of emitted BC was deposited on the Arctic ice. Biomass burning over Northern Eurasia was the dominant source (54%) of the BC deposition in the Arctic. Anthropogenic sources in Northern Eurasia accounted for 24% of BC deposition in the Arctic, while all sources from North America and southern Asia comprised the balance. Seasonally, biomass burning contributed 68% of the BC sources in the Arctic in the spring and 81% in the summer, while anthropogenic sources contributed 73% in the winter and 67% in the fall. About 49% of total BC deposition in the Arctic originated from Asia and only 5% was from Europe. Geographically, in Asia, Siberia was the major source (59%) for the BC deposition in the Arctic, followed by Kazakhstan (18%) and Mongolia (8%).

These results are critical to understanding the contribution of black carbon from biomass burning to accelerated melting of Arctic ice under future climate conditions.